(1) Field of the Invention
The invention relates to a rotary wing rotorcraft having a plurality of propellers, and to a method of optimizing such a rotorcraft.
The invention thus lies in the technical field of rotorcraft, and in particular of rotorcraft having a high forward speed and including at least one lift rotor and at least one propeller.
(2) Description of Related Art
Conventionally, a rotorcraft has a rotary wing that is carried by a fuselage. The rotary wing provides at least some of the lift of the aircraft, and possibly at least some of its propulsion. Such a rotary wing thus includes at least one rotor referred to, for convenience, as a “lift rotor”.
Rotorcraft are also known that have both a rotary wing and a fixed wing carrying at least one propeller arranged beside a fuselage. For example, two propellers may be arranged on either side of the fuselage. For this purpose, each propeller is carried by a half-wing extending transversely from the fuselage of the aircraft.
The propellers then contribute to propelling the aircraft. Furthermore, the propellers can generate torque on the fuselage in order to balance the torque from the main rotor and in order to control yaw movement of the aircraft.
Such a rotorcraft is thus referred to as a “hybrid” aircraft for convenience insofar as a rotary wing is associated with at least one propeller and with at least one fixed wing.
Such hybrid aircraft are particularly advantageous. The use of propellers makes it possible in particular to achieve high forward speeds and to cover considerable distances.
Nevertheless, making a hybrid aircraft can raise difficulties in terms of design dimensions.
The aircraft must present a safety distance between the propellers and the rotary wing, and also between the propellers and the ground. Under such circumstances, it can be difficult to arrange propellers that are suitable for maximizing the performance of the aircraft.
Ground clearance between the ground and propellers that are relatively large may turn out to be marginal in terms of the aircraft having an acceptable roll angle on the ground.
In the process of designing an aircraft, the rotary wing is given dimensions to ensure adequate lift. The rotary wing of a hybrid aircraft (like that of a helicopter for example) then exerts a yaw torque on the fuselage.
In order to compensate that torque, the manufacturer needs to find a good compromise between the diameter of the propellers and the distance between the propellers and the axis of the aircraft. Nevertheless, the blades of a rotary wing tend to flex while they are in use, in particular during stages of take-off and landing. The amount of flexing increases with increasing distance from the axis of rotation of the rotary wing. This phenomenon is additional to the flapping movement of the blades.
Consequently, by locating the propellers of a hybrid aircraft far away from the fuselage, the distance between a propeller and the blades of the rotary wing is reduced. This distance is referred to as “rotor clearance” by analogy with the term “ground clearance”, which designates the distance between a member of the aircraft and the ground when the aircraft is standing on the ground.
A manufacturer thus tends to space each propeller away from the fuselage, while also locating it closer to the ground in order to maintain an acceptable amount of “rotor clearance”.
In order to obtain an acceptable compromise in the design of the aircraft, the manufacturer can thus increase the lift generated by the rotary wing, and can locate the propulsion propellers further away from the fuselage by bringing them closer to the ground.
Nevertheless, by bringing the propellers closer to the ground, the angle of inclination of a straight line passing through a low point of a propeller and a point of contact of the landing gear with the ground is made smaller. In other words, the acceptable roll angle on the ground for the aircraft is made smaller.
Consequently, in order to maintain an acceptable roll angle, the manufacturer may modify the height of the fuselage above the ground in order to increase the ground clearance of the propeller. The roll angle is thus increased.
The manufacturer may also provide outriggers close to the propellers in order to avoid a propeller making contact with the ground.
An outrigger is a piece of landing gear located under a wing. Outrigger technology can lead to difficulties, e.g. requiring the wing carrying the propellers to be reinforced and increasing the overall weight of the aircraft.
In order to compensate for a loss of stability resulting from this increased height, the width of the fuselage is optionally also significantly increased, thereby also leading to an increase in weight.
Nevertheless, the hybrid aircraft then presents a fuselage of large frontal area. For convenience, the term “frontal area” is used to designate the area of the fuselage that comes into contact with air as a result of forward flight. The resulting increase in the drag of the hybrid aircraft then runs the risk of degrading its performance.
In this context, the invention thus lies in the narrow technical field of rotorcraft having both a rotary wing and propellers, and it seeks to avoid such degraded performance.
In the technological background remote from the technical field of the invention, airplanes are known presenting a so-called “push-pull” configuration. It can be understood that an airplane does not form part of the technical field of the invention insofar as an airplane is not subjected to constraints resulting in particular from arranging a rotary wing.
Such an airplane has two propellers arranged one behind the other in an anteroposterior plane of symmetry of the airplane. More precisely, the airplane has a tractor propeller, also referred to as a “forward-mounted” propeller, and a pusher propeller, also referred to as a “backward-mounted” propeller. A tractor propeller faces towards the front of the aircraft while a pusher propeller faces towards the rear of the aircraft. Both propellers then generate thrust towards the front of the aircraft.
That push-pull concept seeks to minimize the drag generated by the two propellers by placing one behind the other. Furthermore, that concept seeks to make it easier to fly the airplane in the event of one of the propellers failing. That concept requires the propellers to be positioned in a central plane in order to avoid destabilizing the aircraft in the event of a failure of one propeller.
Under such circumstances, the push-pull concept does not provide any teaching for solving problems associated with ground clearance and with rotor clearance in a hybrid aircraft.
Reference may also be made to Documents U.S. Pat. No. 1,509,344 and U.S. Pat. No. 5,782,427, for example. Document DE 4 443 731 also describes an engine with twin propellers arranged in the plane of symmetry of an airplane.
Document FR 875 648 describes a multi-engined airplane seeking to leave clear the nose of the fuselage. That airplane has a tail propeller, and possibly two engine-and-propeller units in tandem. That document does not mention the problems of ground clearance and rotor clearance, and it therefore does not lie in the technical field of the invention.
Also known are the following documents: U.S. Pat. No. 1,957,277 and JP 2001/071998.